1. Field of the Invention
The present invention relate to an ink jet recording medium (hereinafter sometimes referred simply to as a "recording medium"). More particularly it relates to an ink jet recording medium comprising a base having on one side thereof a recording layer, on which ink (mostly an aqueous ink) is jetted in dots to form an image. In particular, the invention relates to an ink jet recording medium suitable for formation of a full color image of high resolving power.
2. Description of the Prior Art
An ink jet recording system comprises jetting ink droplets onto a recording medium to form a dot image (inclusive of letters). The system is less noisy than a dot impact type recording system, can be applied to full color printing with ease, and makes high-speed printing possible. The recent advancement of the ink jetting technique has achieved further improvements of recording characteristics, particularly resolving power.
Recording media to be used in the ink jet recording system include plain paper and coated paper.
Coated paper is generally composed of a paper base and a porous ink receiving layer. The ink receiving layer comprises a pigment, a binder, and additives. Pigments having porosity and a low refractive index are used for assuring high ink absorbing properties and a high color density, and amorphous silica is of the most frequent use. Water-soluble resins having satisfactory film forming properties are used as binder for assuring fixing strength and ink absorbing properties, and polyvinyl alcohol (hereinafter abbreviated as PVA) and polymer latices are frequently used. The additives used include cationic resins for improvement of water resistance, photo stabilizers for improvement of light resistance, and fluorescent brightening agents for improvement of whiteness.
Such coated paper provides images with a higher color density and a clearer hue, i.e., higher color reproducibility, than plain paper. Further, a so-called feathering phenomenon (dendrical runs of ink along cellulose fibers) is inhibited to improve dot roundness. Furthermore, the dot diameter can be controlled by selecting the components of the ink receiving layer to further increase the resolving power.
Therefore coated paper is suitable for obtaining color images of high resolving power. However, the ink receiving layer of coated paper tends to cause spreading of jetted ink droplets because of its high water absorbing properties, which has been a limit in further improving the resolving power.
In order to suppress ink's spreading in the ink receiving layer to further improve the resolving power, a recording medium having an ink receiving layer covered with a water-repellent layer has been proposed as disclosed in JP-A-61-89082 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
JP-A-6-55830 discloses a method for improving running properties of a recording medium by coating the back side of a paper base with an aqueous high polymeric resin containing 0.1 to 10% by weight of an aliphatic hydrocarbon lubricant or a metal soap lubricant to a coating weight of 0.4 to 6 g/m.sup.2. The publication has a mention that use of the lubricant in an amount exceeding 10% by weight excessively decreases the coefficient of friction to cause slippage, another running trouble.
Coated paper, when piled up, tends to adhere to each other on account of the smooth surface as compared with plain paper. Ink jet recording apparatus are often equipped with a paper feed tray in which a large number of recording media are put in a pile and from which each recording medium is fed by means of a feed roll. If coated paper is set in a pile in the tray, there is a fear of double feeding (two or more sheets of paper are fed at a time) due to the adhesion or frictional force.
The above-mentioned recording medium having an ink receiving layer covered with a water-repellent layer is apt to gather moisture on the water-repellent layer on being exposed in a high humidity environment and tends to cause blocking (adhesion) when piled up due to adsorbing action of the moisture. Therefore, when the recording media of this type are set in a paper feed tray in a pile, there is a fear of double feeding.